Photoconductive semiconductor switches (PCSS) are known, and generally operate by blocking current while in the off state to maintain voltage in the circuit due to its high resistance, and becoming optically excited while in the on state to become conductive and transfer voltage into a load. PCSS are required for a variety of applications including for example grid switching, power switching, RF generation, and others. Prior work has demonstrated photoconductive switching using Si, GaAs, SiC, and other materials, and various methods of illuminating photoconductive switch material to activate conduction are known.
For example, one known photoconductive switch design uses an edge illumination scheme in an effort to increase optical absorption by lengthening the optical path. This illumination method introduces light through a side (i.e. an “edge facet”) of a rectangular prism-shaped photoconductive material having a generally at profile, with two dimensions (e.g. x and y dimensions) substantially greater than the third dimension (e.g. z dimension), wherein four of the six facets are considered edge facets defined in part by the smaller third dimension and the two opposing wider facets are characterized as electrode connection/contact surfaces where electrodes typically connect to the photoconductive material. While the long path length of such edge illuminated designs gives moderate light utilization capability, most of the light is known to still escape out through the back side of the switch. Furthermore, edge illuminated switches have been known to exhibit surface flashover, field build up at the inactive regions, and other problems. In particular, low breakdown voltage in the on-state is considered to be caused by charge build up at the junction between the metal electrode, the air and the photoconductive material, referred to as the “triple point,” which leads to enhancement of the electric field. The field enhancement occurs in the switch on-state when current is generated in the entirety of the substrate which forces the current in the region outside of the metal contact to concentrate at the electrode edge, resulting in field enhancement and reduced breakdown voltage.
Another known photoconductive switch design uses axial illumination, whereby light is introduced normal to an electrode contact-surface of a rectangular prism-shaped photoconductive material so that all the carriers are confined within the metal electrode. While this axially-pumped switch design enabled very high voltage due to the minimization of triple junction field enhancements, it typically has very low light utilization on the order of 1% utilization of the light due to the low absorption coefficient and short optical path length. Consequently, this design has required very high laser fluence for operation, which in turn can degrade the metal contacts on the top and bottom of the switch and can damage the fiber optics delivering the light to the switch,
Bulk excitation of the switch with sub-bandgap light has the advantage of thicker (˜1 mm) devices that can hold voltages in excess of 30 kV, and uniform generation of charge carriers over entire thickness, necessary to turn the switch “on”. However, uniform conductivity in the bulk has typically come at the cost of low absorbance, and therefore a photoconductive switch with improve optical absorption is still needed.